The present invention relates to the field of shearing interferometers for measuring the relative phase shifts at points in the wavefront which are indicative of wavefront distortion.
In U.S. Pat. No. 3,829,219 to Wyant, a shearing interferometer is described having first and second diffraction gratings having slightly different spatial frequencies (e.g., 290 and 310 lines/mm) for producing angularly displaced beams having a region of overlap within which a lateral shearing interferogram is produced, which is a measure of wavefront distortion. Such a device is useful for either direct visual observation of the shearing interferogram or may be employed in connection with electronic phase measurement arrangements for indicating and correcting wavefront distortion. In this patent a pair of ruled diffraction gratings are translated mechanically, transverse to the direction of light propogation.
In U.S. Pat. No. 3,923,400 to Hardy, a shearing interferometer system is disclosed which, like the aforesaid patent, includes a similar two-frequency grating positioned near the focal plane of the wavefront to be examined. The translation of the two-frequency grating also produces modulation of the interference pattern. As in the arrangement of Wyant, the diffraction gratings are slightly different in spatial frequency which causes the production of two diffracted cones of light at two slightly different angles, which however, have a common region of overlap. The result is the formation of a shearing interferogram in the region of overlap, and the measurement of the fringe spacing between fringes within the overlap will reveal or measure the departure of the wavefront from its ideal shape.
Unlike the first mentioned patent to Wyant, the Hardy patent teaches the use of two sets of two diffraction gratings, each set formed at right angles with respect to the other, on a single piece of photographic film. The film is moved in a direction of 45.degree. relative to the orientation of the four diffraction gratings on the film which in turn causes modulation of the sheared interferograms, allowing the techniques of electronic A.C. detection to be applied. The orthogonal grating patterns cause shearing in x and y in order to measure wavefront distortion in the two dimensional domain.
In both of these patents, incorporated by reference herein, the gratings are photographically recorded on film or the like, and are moved by means of a drive system such as a moving loudspeaker coil transducer.
It is an object of the present invention to provide a new and improved shearing interferometer for examining wavefront distortion.
It is a further object of the invention to produce a shearing interferometer whereby the diffraction grating spatial frequencies may be electronically changed with great ease and rapidity to provide changing light deflection to in turn provide sequential scanning of the entire wavefront under examination in both x and y directions, to measure wavefront aberrations. Two linear arrays of photodetectors may be employed, perpendicular to each other, rather than employing an x-y array which requires many more detectors. For example 10 detectors could be positioned in one dimension in x, and 10 in y rather than the 200 detectors required by two 10.times.10 rectangular arrays.
It is also an object of the invention to readily change the amount of shear between the two diffracted cones of light merely by electronically changing the frequency of one control signal relative to another which changes the difference between the spatial frequencies of the two gratings.
It is yet a further object of the invention to provide a system for measuring wavefront aberrations in both x and y without the use of an additional beam splitter and focusing lens.
It is yet a further object of the invention to eliminate the prior art fixed frequency film gratings together with the drive mechanisms therefor.